<!DOCTYPE html>
<html>
<head lang="en">
    <meta charset="UTF-8">
    <title></title>
    <style>
        div{
            width: 200px;
            height: 70px;
            background-color: blue;
            transition:height 0.25s;
            -webkit-transition: height: 0.25s;
        }

        div:hover{
            height: 500px;
        }
    </style>
</head>
<body>
<div>12121</div>
</body>
</html>